9630285 Crossey Understanding the linkage between dissolved redox-sensitive species and solid reactive phases in an aquifer matrix is crucial to the prediction of nutrient pathways, biogeochemical cycling of carbon, and contaminant transport in ground water systems. In many alluvial aquifers redox-sensitive metals play an important role in terminal electron-accepting processes. Because these metals are critical to aquifer energetics and biogeochemistry, this research focuses on the temporal and spatial controls and redox-sensitive metal mobility in a shallow alluvial aquifer influenced by surface-subsurface water exchange. The study comprises two Research Elements. Research Element I will examine temporal/seasonal controls on redox-sensitive metal mobility. Through systematic surface-subsurface water and sediment sampling and analysis; aquifer characteristics, aqueous geochemistry, and biological functioning will be identified at Rio Calaveras (a heavily-instrumented, unperturbed site in the Jemez Mountains of New Mexico). The shallow alluvial fill will be characterized with respect to fine-scale aquifer structure (stratigraphic elements), sediment geochemistry (e.g., extractable metals, organic matter content, and extractable carbon), and aqueous biogeochemistry (dissolved gases, organic and inorganic solutes). Bi-monthly sampling of an extensive well network will permit measurement of dynamic geochemically, and biologically to seasonal and precipitation event perturbations. Observation of several complete cycles are assured over the time period of the study. Multi-level water sampling instrumentation will allow the high vertical resolution necessary to quantify links among sediment characteristics (bioavailable carbon and metals), aqueous geochemistry, and microbial functioning. Research Element 2 focuses on spatial heterogeneity within the alluvial aquifer, and poses a series of hypotheses and tests to assess the control on metal mobility exerted by (a) position along gro undwater flow paths, (b) chelation by dissolved organic carbon species, and (c) microbially-mediated iron reduction reactions. Hydrologic controls will be assessed through sampling geochemical parameters along flow paths (defined by an existing site-specific hydrologic model) and through application of geochemical models (for aqueous speciation and reaction path modeling). Controlled laboratory experiments will be conducted using sediment incubations to distinguish abiotic (chelation) and biotic (microbial reduction) mechanisms of iron mobilization. Aquifer heterogeneity will be a primary organizing factor in the experimental design. This proposal was submitted in response to the Environmental Geochemistry and Biogeochemistry solicitation, and the project is being jointly supported by the Divisions of Earth Sciences and Divisions of Ocean Sciences (Geosciences).
